This invention relates to a method and apparatus for modifying and converting full-time four-wheel drive vehicles for optional two-wheel drive operation.
Mechanisms for equipping motor vehicles for four-wheel drive operations are well known. Typically, the drive shaft from the transmission of the vehicle is connected to a transfer case which distributes power to two output shafts, one to the rear axle and wheels and one to the front. For vehicles designed to operate only in rough offroad terrain, undifferentiated power to the front and the rear output shafts is acceptable; that is, the transfer case gearing can be used in fixed relationship to drive the front and rear axles at the same speed.
An undifferentiated drive mode for the transfer case is, however, unsuitable for the vast majority of uses, because for highway driving it is undesirable to force both axles to rotate at identical speeds. Forcing such performance may cause excessive tire wear, build up undesirable stresses in the drive chain, detract from fuel efficiency, make cornering unstable or difficult, and create other handling and safety problems. For reasons similar to that for using a differential on each axle to permit differentiated drive to the left and right wheels, it is thus also desirable to provide differentiated power to the front and rear axles. To achieve this, differential gearing is provided in the transfer case to permit the front and rear output shafts to be driven at different speeds in accordance with the relative demands of the two axles. (Differential gears are, of course, also provided for the front and rear axles to permit the left and right wheels of each axle to receive differentiated power distribution and rotate at different speeds.)
One such prior art transfer case is shown and described in U.S. Pat. No. 3,848,691 to Dolan, issued Nov. 19, 1974, which is incorporated herein by reference. Therein is shown a transfer case with a differential which can be locked or unlocked to prevent or permit rotation of the pinion gears, depending on whether the operator desires locked four-wheel drive operation (with the front and rear axles rotating at the same speed) or differentiated four-wheel drive operation (with the front and rear axles subject to different speeds). This type of transfer case has been widely adopted and used in hundreds of thousands, or millions, of four-wheel drive vehicles that are still in service. However, it is subject to a substantial deficiency relative to the need for fuel-efficient motor vehicles, namely, it is incapable of use for two-wheel drive operation. Given that the vast majority of four-wheel drive vehicles operate a great deal of the time in cities or on highways where four-wheel drive is unnecessary, it is extremely desirable to be able to convert them for optional two-wheel drive operation, since this can improve fuel efficiency by as much as 30% in some circumstances.
It is known to adapt the front wheels of four-wheel drive vehicles with optional locking-unlocking hubs to disengage the wheels from the front axle, when desired. By allowing the front wheels to "free-wheel", energy can be saved. These hubs, however, cannot be disengaged when used with a transfer case in differential mode of operation, because under such mode all, or substantially all, the power will be delivered to the front axle, which will simply spin without turning the disengaged front wheels. Since the differential gearing of the transfer case will deliver virtually no power to the rear axle, the vehicle will be immobilized.
On the other hand, it is possible to disengage the front wheels from their axle if the transfer case is operated in locked mode, so that equal rotation is imparted to both the forward and rear output shafts. This, however, is highly inefficient since a substantial portion of engine power will be wasted in turning the front output shaft, differential, and front axle, even though they perform no power function when the front wheel hubs are disengaged.
There is thus a great need for an inexpensive method and means for converting certain full-time four-wheel drive vehicles for optional two-wheel drive operation, and it is an object of the present invention to provide a method and means for such conversions. It is a further object to provide a method and means which are relatively easy and fast to employ by anyone of ordinary skill in the art without the necessity for expensive and complicated tools or equipment. These and other objects will be apparent from the following description of the invention.